[llvm] [LV] Use wide lane masks as the canonical form when tail-folding & interleaving (PR #209484)

[Kerry McLaughlin via llvm-commits]

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@@ -1104,6 +1108,32 @@ Value *VPInstruction::generate(VPTransformState &State) {
 
     return Result;
   }
+  case VPInstruction::ExtractVectorForPart: {
+    Value *Src = State.get(getOperand(0));
+    Type *DstTy = VectorType::get(getScalarType(), State.VF);
+    unsigned Part = cast<VPConstantInt>(getOperand(1))->getZExtValue();
+
+    if (Src->getType() == DstTy)
+      return Src;
+
+    // If the VF is scalar & this is an extract of an active lane mask,
+    // generate an ICMP directly.
+    VPValue *Start, *TC;
+    if (State.VF.isScalar() &&
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kmclaughlin-arm wrote:

Moved this optimisation to `replaceMaskWithCompare`, which is now called from `optimizeForVFAndUF`.

https://github.com/llvm/llvm-project/pull/209484